1. Field of Invention
The invention relates to materials and methods for commercially preparing single walled carbon nanotubes. More specifically, this invention relates to material comprising complex oxides, which, when further processed, are viable activated catalysts for carbon fibril-containing products. These products exhibit both a Raman spectrum and characteristic transmission electron micrographs known to indicate the presence of single walled carbon nanotubes.
2. Description of the Related Art
This invention lies in the field of carbon nanotubes (also known as fibrils). Carbon nanotubes are vermicular carbon deposits having diameters less than 1.0μ, preferably less than 0.5μ, and even more preferably less than 0.2μ. Carbon nanotubes can be either multi walled (i.e., have more than one graphene layer more or less parallel to the nanotube axis) or single walled (i.e., have only a single graphene layer parallel to the nanotube axis). Other types of carbon nanotubes are also known, such as fishbone fibrils (e.g., wherein the graphene layers are arranged in a herringbone pattern, compared to the tube axis), etc. As produced, carbon nanotubes may be in the form of discrete nanotubes, aggregates of nanotubes (i.e., dense, microscopic particulate structure comprising entangled carbon nanotubes) or a mixture of both.
Carbon nanotubes are distinguishable from commercially available continuous carbon fibers. For instance, diameter of continuous carbon fibers, which is always greater than 1.0μ and typically 5 to 7μ, is far larger than that of carbon nanotubes, which is usually less than 1.0μ. Carbon nanotubes also have vastly superior strength and conductivity than carbon fibers.
Carbon nanotubes also differ physically and chemically from other forms of carbon such as standard graphite and carbon black. Standard graphite, because of its structure, can undergo oxidation to almost complete saturation. Moreover, carbon black is an amorphous carbon generally in the form of spheroidal particles having a graphene structure, such as carbon layers around a disordered nucleus. On the other hand, carbon nanotubes have one or more layers of ordered graphitic carbon atoms disposed substantially concentrically about the cylindrical axis of the nanotube. These differences, among others, make graphite and carbon black poor predictors of carbon nanotube chemistry.
It has been further accepted that multi walled and single walled carbon nanotubes are also different from each other. For example, multi walled carbon nanotubes have multiple layers of graphite along the nanotube axis while single walled carbon nanotubes only have a single graphitic layer on the nanotube axis.
The methods of producing multi walled carbon nanotubes also differ from the methods used to produce single walled carbon nanotubes. Specifically, different combinations of catalysts, catalyst supports, raw materials and reaction conditions are required to yield multi walled versus single walled carbon nanotubes. Certain combinations will also yield a mixture of multi walled and single walled carbon nanotubes.
As such, two characteristics are often examined in order to determine whether such process will be commercially feasible for the production of a desired carbon nanotube on an industrial scale. The first is catalyst selectivity (e.g., will the catalyst yield primarily single wall carbon nanotubes or primarily multi-walled carbon nanotubes or other forms of carbon products?). Products of poor selectivity catalysts are contaminated with non-nanotube carbon and usually require purification, conventionally accomplished by selective oxidation of the non-nanotube carbon. Not only does this add an additional and costly process step, but it also results in some loss of the desired single walled carbon nanotubes and may further lead to unintended functionalization of the recovered purified single walled carbon nanotube product. Additionally, mixtures of single and multi walled carbon nanotubes can be difficult to separate. Thus, if single walled carbon nanotubes are desired, the co-produced multi walled carbon nanotubes can be an impurity reducing the usefulness of the product. Thus, a selectivity to single walled carbon nanotubes of at least 50% is preferred. A selectivity of at least 80% to single walled carbon nanotubes is more preferred.
The second is catalyst yield (e.g., weight of carbon product generated per weight of catalyst used). Low yields, i.e. less than 1.0 gram of carbon product per gram of catalyst, lead to a need for extensive purification and are thus undesirable. When both purification and catalyst removal are needed, it is desirable that both operations be accomplished in a single step.
It is most advantageous to produce single walled carbon nanotubes in a yield and at a selectivity such that the product can be used without further processing. Commercial processes are often distinguished incidental observations of single walled carbon nanotubes based on a more robust combination of yield and selectivity. Thus, the combination of a selectivity greater than 50% single walled carbon nanotubes and a yield greater than 1.0 gm carbon/gm catalyst is preferred. Even more preferred are selectivities greater than 80% single walled carbon nanotubes combined with yields greater than 2.0 gm carbon/gm catalyst.
Single-wall nanotube catalyst selectivity can be measured through evaluation of Raman spectra signatures of fibril-containing products, which are informative for differentiating single (and perhaps, double)-walled nanotubes from multi-walled tubes. E.g., “Diameter-Selective Raman Scattering from Vibrational Modes in Carbon Nanotubes,” Rao, A M et al, Science, vol. 257, p. 187 (1997); Dresselhaus, M. S., et al., “Single Nanotube Raman Spectroscopy,” Accounts Of Chemical Research, vol. 35, no. 12, pp. 1070-1078 (2002), both hereby incorporated by reference. For example, a sample having sufficiently small diameter nanotubes to be single-walled has a Raman spectrum exhibiting: “radial breathing mode” (RBM) peaks between 150 and 300 wave numbers, the area under the RBM peaks at least 0.1% of the area under a characteristic G band peak, and the intensity of the G band peak at least twice that of a characteristic D band peak (G/D of at least 2.0).
The following multi-walled tube (MWNT) process references are hereby incorporated by reference: Baker and Harris, Chemistry and Physics of Carbon, Walker and Thrower ed., Vol. 14, 1978, p. 83; Rodriguez, N., J. Mater. Research, Vol. 8, p. 3233 (1993); Oberlin, A. and Endo, M., J. of Crystal Growth, Vol. 32 (1976), pp. 335-349; U.S. Pat. No. 4,663,230 to Tennent et al.; U.S. Pat. No. 5,171,560 to Tennent et al.; Iijima, Nature 354, 56, 1991; Weaver, Science 265, 1994; de Heer, Walt A., “Nanotubes and the Pursuit of Applications,” MRS Bulletin, April, 2004, U.S. Pat. No. 5,456,897 to Moy et al, U.S. Pat. No. 6,143,689 to Moy et al, and U.S. Pat. No. 5,569,635 to Moy et al.
Processes for making single-walled carbon nanotubes (SWNT) are also known. E.g., “Single-shell carbon nanotubes of 1-nm diameter”, Iijima, S, and Ichihashi, T. Nature, vol. 363, p. 603 (1993); “Cobalt-catalysed growth of carbon nanotubes with single-atomic-layer walls,” Bethune, D S, Kiang, C H, DeVries, M S, Gorman, G, Savoy, R and Beyers, R Nature, vol. 363, p. 605 (1993); U.S. Pat. No. 5,424,054 to Bethune et al.; Guo, T., Nikoleev, P., Thess, A., Colbert, D. T., and Smalley, R. E., Chem. Phys. Letters 243: 1-12 (1995); Thess, A., Lee, R., Nikolaev, P., Dai, H., Petit, P., Robert, J., Xu, C., Lee, Y. H., Kim, S. G., Rinzler, A. G., Colbert, D. T., Scuseria, G. E., Tonarek, D., Fischer, J. E., and Smalley, R. E., Science, 273: 483-487 (1996); Dai., H., Rinzler, A. G., Nikolaev, P., Thess, A., Colbert, D. T., and Smalley, R. E., Chem. Phys. Letters 260: 471-475 (1996); U.S. Pat. No. 6,761,870 (also WO 00/26138) to Smalley et al; “Controlled production of single-wall carbon nanotubes by catalytic decomposition of CO on bimetallic Co—Mo catalysts,” Chemical Physics Letters, 317 (2000) 497-503; U.S. Pat. No. 6,333,016 to Resasco et al.; “Low-temperature synthesis of high-purity single walled carbon nanotubes from alcohol,” Maruyama et al Chemical Physics Letters, 360, pp. 229-234 (Jul. 10, 2002). These articles and patent documents are hereby incorporated by reference. Currently known processes for forming single-walled tubes are unable to reach industrially acceptable levels of selectivity and yield under commercially viable reaction conditions.
Recent literature contains disclosures describing the benefits of using catalytic precursors that comprise solid solutions of transition metal oxide(s) and non-reducible (at practical temperatures) oxides. These solid solutions of mixed oxides must be calcined at relatively high temperatures to avoid the presence of non-soluble simple oxide phases. Bacsa, R. R. et al., Chem. Phys. Letters 323: 566-571 (2000) and J. Am. Ceram. Soc., 85: 2666-69 (2002), both incorporated by reference, describe catalysts made by the selective reduction (T>800° C.) in H2/CH4 of “solid solutions between one or more transition metal oxides and a non-reducible oxide such as Al2O3, MgAl2O4 or MgO.” The solid solutions were made by combustion synthesis, employing combustion of both precursors and a fuel (typically urea). Both transmission electron micrographs and Raman spectra showed the presence of a mixture of single-walled/double walled tubes and a substantial amount of non-tubular amorphous products. Flahaut, et al., J. Materials Chemistry, 10: 249-252 (2000) describes the same catalyst synthesis as above except giving combustion synthesis temperature as “usually >800° C.”.
Coquay, et al., J. Phys Chem B, 106: 13199 (2002) and Coquay, et al. J. Phys Chem B, 106: 13186 (2002) both identify that the use of oxide phase Co3O4 catalyzes a yield of thick nanofibers. This was a shortcoming of previous flame synthesis methods in making single-walled nanotube catalysts. Flame-synthesized Mg1-xFexO solid solutions are found to catalyze formation of single-walled nanotubes, while A2BO4— like particles tend to yield only thick nanofibers. The electron micrographs of product made from flame-synthesized Mg1-xFexO solid solutions reveal that these catalysts only occasionally yield form SWNTs, rather than selectively, thus yielding a few SWNTS along with a broad spectrum of other carbonaceous products. Coquay, et al., J. Phys Chem B, 109:17813 (2005) studied mixed solid solutions of Fe, Co and Mg prepared by the same high temperature combustion method and reduced in H2/CH4 at 1000° C. Again, only trace yields of mixed single walled and multi walled tubes contaminated with substantial amounts of non-tubular amorphous carbon were observed. This paper concluded that “the highest CNT quantity and carbon quality are eventually obtained by reduction of iron free Mg0.9Co0.1O solid solution.”
Wang and Ruckenstein, Carbon, 40: 1911-1917 (2002) disclosed and characterized a range of Co/Mg/O catalysts with differing stoichiometries and calcining temperatures used in their preparation. They found that the A2BO4 phase only forms at calcining T<700° C. but yielded no filamentous product. When catalysts were calcined at T=900° C. and reacted with methane at the same temperature, microscopic analysis revealed formation of filamentous carbon, but not single-walled nanotubes.
The references cited above, while employing mixed metals as catalysts, all disclose and specifically conclude that solid solutions (in contrast with complex oxide phase material) are favored to generate either tubular or filamentous carbon products. A solid solution is a solid-state solution of one or more solutes in a solvent. Such a mixture is considered a solution rather than a compound when the crystal structure of the solvent remains unchanged by addition of the solutes, and when the mixture remains in a single homogeneous phase. Some mixtures will readily form solid solutions over a range of concentrations, while other mixtures will not form solid solutions at all. The propensity for any two substances to form a solid solution is a complicated matter involving the chemical, crystallographic, and quantum properties of the substances in question. Generally, solute and solvent should have similar atomic radii (15% or less difference), same crystal structure, similar electronegativities, and similar valence state.
On the other hand, a complex oxide is a general definition of oxides or minerals that have specific crystallographic structures and chemical stoichiometry such as rock salt (ABO2, A2BO3), spinel (A2BO4) or perovskite-type (ABO3). It is usually made of at least two metal oxides (A and B), for example, perovskite CaTiO3. Under certain circumstances, complex oxide can be converted into solid solution by high temperature process.
Due to the different crystallographic structure, these two materials can usually be distinguished by using crystallographic characterization technique such as diffraction of electron, neutron or X-ray.
There is a need for a method for producing single walled carbon nanotubes with industrially acceptable levels of activity, selectivity and yield under commercially viable reaction conditions. As discussed above, none of the prior art discloses such a methodology; discovery of an acceptable process remains elusive despite an ongoing worldwide search to develop it.